1. Field of the Invention
The present invention relates to new urethane acrylates and to their use as a binder component in coating compositions that are curable with high-energy radiation.
2. Description of the Prior Art
Urethane acrylates are known as coating binders that are curable with high-energy radiation. A review is given in P. K. T. Oldring (Ed.), Chemistry & Technology of UV & EB Formulations for Coatings, Inks & Paints, Vol. 2, 1991, SITA Technology, London, pages 73-123. Urethane acrylates constitute high-grade coating binders for substrates, such as wood, metal and plastics and for mineral substrates.
Very different polyisocyanates can be used for the production of urethane acrylates. (Cyclo)aliphatic polyisocyanates are used for the production of urethane acrylates when it is necessary for the resulting coatings to be resistant to yellowing and weathering. Although hexamethylene diisocyanate is also disclosed as a suitable diisocyanate, e.g., in DE-A 2,115,373, it can be stated that this is a purely hypothetical disclosure. This diisocyanate cannot be used in the formulations described in this reference due to the tendency of the resulting urethane acrylates to crystallize. To be useful the lacquer binders have to be liquid under normal conditions.
DE-A 4,027,743 describes urethane acrylates prepared from hexamethylene diisocyanate, which are modified with ester alcohols such that they are liquid at room temperature. However, at temperatures below 10.degree. C., these high- to medium viscosity products become turbid.
It is known from DE-A 4,232,013 that the following methods have been used to suppress the crystallization of urethane acrylates based on hexamethylene diisocyanate: the use of mixtures with other diisocyanates e.g. isophorone diisocyanate, the incorporation of alcohol components containing ester groups, and the incorporation of sterically-hindered mono- or dihydric alcohols that have a branched molecular structure. All of these measures restrict the options for production, and thus restrict the possibility of varying other properties, and generally result in high raw material costs.
Higher functionality polyisocyanates prepared from hexamethylene diisocyanate may also be used to prepare urethane acrylates. Thus, DE-A 3,737,244 describes urethane acrylates which are prepared from polyisocyanates containing isocyanurate groups. The higher functionality of these products results in coatings having better resistance properties. However, the tendency of these products to crystallize is even greater than for urethane acrylates prepared from hexamethylene diisocyanate.
An object of the present invention is to provide new, low-viscosity urethane acrylates which are resistant to yellowing, do not crystallize even at temperatures below 10.degree. C. and are prepared from polyisocyanates having a functionality greater than 2.
Surprisingly, this object may be achieved in accordance with the present invention by using polyisocyanates containing iminooxadiazine dione groups to prepare urethane acrylates that exhibit a considerably reduced tendency to crystallize.